A fundamental design challenge in creating a memory cell of an electrically erasable programmable read only memory (EEPROM) device is to use a controllable and reproducible electrical effect that has sufficient non-linearity so that the memory cell (1) can be written to (or erased) at one voltage in less than one millisecond (1 ms) and can be read at another voltage, and (2) the data within the memory cell must remain unchanged for more than ten (10) years.
Prior art stacked/split gate EEPROM technology requires (1) special multi-polysilicon materials, (2) different gate oxide thicknesses, and (3) modified doping profiles. These prior art requirements create process complexity and high cost when embedded into a complementary metal oxide semiconductor (CMOS) process.
Some of the disadvantages of earlier prior art EEPROM memory cells include low programming speed, high power consumption, high programming voltages, over-erase problems, and high processing complexity. Many of these disadvantages have been overcome by the development of new types of EEPROM Flash memory technology.
For example, research center IMEC of Leuven, Belgium has developed a proprietary EEPROM Flash memory technology under the name HIMOS®. The name HIMOS® is a registered trademark of IMEC. The name HIMOS® stands for High Injection (efficiency) Metal Oxide Semiconductor (MOS). The HIMOS® EEPROM Flash memory technology overcomes most of the drawbacks and problems associated with many of the Flash memory concepts in current use. In addition, the HIMOS® EEPROM Flash memory technology is more compatible with conventional complementary metal oxide semiconductor (CMOS) manufacturing processes.
FIG. 1 illustrates a schematic diagram of a prior art HIMOS® EEPROM Flash memory cell 100. Memory cell 100 comprises a floating gate (FG) 110 that is shown as a shaded area in FIG. 1. Memory cell 100 also comprises program gate (PG) 120 and control gate (CG) 130, source 140 and drain 150 arranged in the configuration shown in FIG. 1.
FIG. 2 illustrates a schematic diagram of a cross sectional view of the HIMOS® EEPROM Flash memory cell 100 shown in FIG. 1 taken along the line A-A′. FIG. 3 illustrates a schematic diagram of a cross sectional view of the HIMOS® EEPROM Flash memory cell 100 shown in FIG. 1 taken along the line B-B′.
The programming voltages shown in FIGS. 2 and 3 are for 0.35 μm technology. The source 140 is grounded and the drain 150 is coupled to a voltage of three and three tenths volts (3.3 V). The control gate (CG) 130 is coupled to a voltage of nine tenths of a volt (0.9 V). The program gate (PG) 120 is coupled to a voltage of nine volts (9 V) for supplying the necessary voltage to program the memory cell 100.
A significant problem area of the HIMOS® EEPROM Flash memory technology relates to its erase operation. Because the HIMOS® EEPROM Flash memory technology has a triple gate structure, there are three different possible modes for carrying out the erase operation. The three modes are Drain Erasure, Interpoly Erasure, and Channel Erasure. Each of the three erasure modes has its own disadvantages.
Drain Erasure mode. The standard erasure mode is drain-side erasure by Fowler-Nordheim (FN) tunneling. Fowler-Nordheim tunneling is carried out by applying a small to moderate value of voltage (3.3 V to 4.5 V) to the drain and a moderate value of negative voltage (−6.0 V to −5.0 V) to the program gate and to the control gate. By also applying the negative voltage to the control gate, the additional capacitance between the control gate and the floating gate helps to build up the necessary tunneling field, and the erase voltage is significantly reduced. Using two gates (the control gate and the floating gate) during erasure permits significant reduction in the value of required erase voltage.
The Drain Erasure mode has serious reliability concerns. The drain erasure configuration is also the same configuration for generating hot holes. This configuration (1) causes oxide damage and degrades cycling performance, and (2) over-erases as a result of the injection of extra holes into the floating gate in addition to the electrons that tunnel out of the floating gate.
Interpoly Erasure mode. Erasure can also be accomplished by interpoly conduction, which is established by applying a moderate positive voltage to the control gate, eventually combined with a moderate negative voltage to the program gate. In CMOS processes the interpoly oxide integrity is not as good as the integrity of thermally grown oxide. Interpoly Erasure causes memory window early closure and erase time push out. The leakage current is also high and leads to high temperature retention degradation. Except for split gate flash memory, no other mainstream flash technology uses an interpoly erasure scheme. In split gate flash technology, a special injector has to be created to carry out the Interpoly Erasure process. The creation of the structure of the special injector for the memory cell requires complicated and expensive processes.
Channel Erasure mode. In the Channel Erasure mode a positive voltage is applied to the P-well of the memory array in order to avoid the band-to-band tunneling (and the correlated hot hole injection) that occurs in the drain-side erase mode. The Channel Erasure mode permits low-power erasure at the expense of adding one more processing step in the manufacturing process (i.e., the addition of an N-well for memory array isolation).
Of the three erasure modes for the HIMOS® flash memory cell, the Channel Erasure mode has the best reliability performance. However, its superior reliability performance comes with the drawback of requiring an additional N-well for memory array isolation. The additional N-well requires an additional mask step and therefore adds additional cost to the manufacture of the HIMOS® flash memory cell.
Therefore, there is a need in the art for a system and method that is capable of solving the performance problems described above that are exhibited by the HIMOS® EEPROM Flash memory technology. In particular, there is a need in the art for a system and method for providing an improved EEPROM flash memory cell having an increased level of reliability for the erase process without having to provide an additional N-well in the memory cell.
The present invention provides a new EEPROM memory cell structure that comprises an erase capacitor connected to the program gate (PG) and the control gate (CG) of a HIMOS® EEPROM Flash memory cell. The erase capacitor of the present invention is implemented by tying together the source and drain and N-well of a P-type metal oxide semiconductor (PMOS) transistor in a standard CMOS manufacturing process. The gate of the PMOS transistor is connected to the floating gate (FG) of the memory cell and becomes part of the floating gate. A positive voltage bias is applied to the erase capacitor to carry out a channel erasure process in the EEPROM memory cell.
The EEPROM memory cell of the present invention provides an increased level of reliability for the erase process without requiring an additional process step to be added in the manufacture of the EEPROM memory cell. Furthermore, by optimizing the capacitive coupling between the PMOS transistor and the EEPROM memory cell, the erase voltage can be significantly decreased. This reduces the size of the peripheral circuitry that is used to generate the high voltage and, consequently, improves the efficiency of the memory cell array.
Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.
Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as to future uses, of such defined words and phrases.